1) Field of the Invention
This invention pertains to the field of measurements of refractive errors in an optical system, and more particularly to systems and methods for compiling a tomographic mapping of the refractive errors in an optical system such as the eye.
2) Description of the Related Art
Measurements of aberrations of the eye are important for the diagnosis of visual defects and acuity. There are a growing number of ways that aberrations can be corrected using both surgical and non-surgical means. These methods rely on accurate, precise measurements of the whole ocular system so that patients may be screened, the corrective means applied and tested, and followed up as appropriate. In addition, an enhancement to the accuracy and precision of ocular measurements may lead to improved methods for correcting visual defects, and for identifying patients in need of care.
There are a number of existing methods used to measure the performance of the ocular optical system. The best established are psychophysical methods, which rely on subjective patient feedback to for the parameters of the measurement. The oldest of these is the phoropter or trial lens method. This technique relies on a trial and error method to identify the required correction. There are psychophysical techniques for measuring visual acuity, ocular modulation transfer function, contrast sensitivity and other parameters of interest. Such techniques are disclosed, for example, in DAVID A. GOSS AND ROGER W. WEST, INTRODUCTION TO THE OPTICS OF THE EYE (2002).
In addition to the subjective methods, there are number of objective means for measuring the performance of the ocular system. These include corneal topography, wavefront aberrometry, corneal interferometry, auto-refraction, and numerous other means for measuring the eye. These methods may be summarized as described below.
The surface shape and thickness of the cornea are extremely important information for laser vision correction surgery, inter-ocular contacts, radial keratotomy, and other surgical repair and correction schemes. Comeal topography can measure the surface shape of the cornea. Corneal topography can be used to measure the deviation of the corneal shape from the ideal shape required for perfect vision. There are several commercial instruments that use different methods to accomplish this. Many of these methods operate on the cornea directly, and thus its thickness, shape and other parameters are critical to obtaining good results. U.S. Pat. Nos. 4,838,679, 5,062,702, 5,822,035, and 5,920,373 to BILLE disclose mapping the cornea of an eye using a variety of methods. The cornea has enough of a difference in index of refraction between the front and rear surfaces that it is possible to also measure the corneal thickness.
However, the cornea only partially contributes to the optical errors of the ocular system. Many other elements, such as vitreous fluid and the crystalline lens may also be significant factors that are not accounted for by corneal topography.
Another instrument for objectively determining the refraction of the eye is the auto-refractor. The auto-refractor uses one of various means to automatically determine the required corrective prescription. This may consist of the projection of one or more spots or patterns onto the retina. Through adjustment of various optical elements in the auto-refractor instrument, the required correction is automatically detected. Numerous auto-refractors have been developed and are in common clinical use. Examples may be found in U.S. Pat. Nos. 3,819,256 and 4,021,102.
However, the accuracy of the refraction is often suspect, and eye doctors rarely use this information without further refinement. The basic problem with the auto-refractor is that it measures only lower order components of the aberrations, such as the spherical and astigmatic errors. Higher order aberrations are not accounted for by the auto-refractor. Only the average performance of the optical system is measured by the auto-refractor.
Recently, there has been attention focused on treatment of the eye as an optical system. This has lead to the application of methods for measuring the eye that have previously been used for other optical systems, e.g., interferometry, Shack-Hartmann wavefront sensing. These techniques are extremely powerful because they measure the complete aberrations of the eye""s optical system.
In wavefront aberrometry, a spot is projected on the retina of the eye and then the resulting scattered light is measured with an optical system. The full, end-to-end integrated line of sight, measurement of the aberrations of the eye is obtained. Thus, wavefront aberrometry can be used to measure the full aberration content of the optical system of the eye from end to end.
This additional information allows researchers and clinicians to measure non-symmetric, non-uniform effects that may be affecting vision. In addition, the information can be linked directly to many of the various corrective means to provide greatly improved vision for many patients.
U.S. Pat. No. 5,777,719 to WILLIAMS describes the application of Shack-Hartmann wavefront sensing and adaptive optics for determining the ocular aberrations to make a super-resolution retina-scope. This information is then used to make better contact lenses, inter-ocular lenses and other optics as disclosed in U.S. Pat. No. 5,949,521 to WILLIAMS. PCT patent publication WO 00/10448 by AUTONOMOUS TECHNOLOGIES discloses refined methods for projecting the light beam onto the retina. U.S. Pat. No. 6,007,204 to ALLYN WELCH discloses a simplified hand held unit. Commonly owned, co-pending U.S. patent application Ser. No. 09/692,483 (Attorney Docket No. WFS.006) to NEAL ET AL. discloses an integrated instrument that uses an improved projection system to minimize the size of the spot on the back of the eye, and thus allow much higher resolution wavefront sensing.
As described above, corneal topography can be used to measure the deviation of the corneal shape from the ideal shape required for perfect vision, and wavefront aberrometry can be used to measure the full aberration content of the optical system from end to end. However, for most surgical (and some non-surgical) procedures, knowledge of both the corneal shape and the wavefront aberrations is needed. This can be accomplished by measuring the same eye successively with both a wavefront aberrometer and a corneal topographic instrument, or by making these measurements with a combined instrument. U.S. Pat. No. 6,050,687 to BILLE discloses a method for integrating both corneal topographic and wavefront aberration functions into a single device.
The objective of such a combined instrument is to identify not only the aberration content of the eye, but to separate the effects of the various contributors. Roughly 30% of the aberration is known to be due to corneal shape. Thus the remaining 70% is due to other, buried structures. Wavefront aberrometry does not provide a measure of the three-dimensional structure of the index of refraction field. Combining wavefront aberrometry with corneal topography allows the user to determine the contribution due to the surface, but does not identify any other source.
What is really needed is a means for measuring not only the aberrations of individual structures, but also the full three-dimensional structure of an eye or other optical system.
The measurement of three-dimensional structures interior to a media is a difficult, if often studied, problem. For human biological systems, a non-invasive procedure is required. This limits the methods that are available. Since the eye can be probed only from the front (without extensive surgical methods), there is a natural limit to what can be measured. This problem is encountered in x-ray radiology, where internal organ or skeletal structure is studied. There are a number of techniques that have been applied to this field, the most notable of which are nuclear magnetic resonance (NMR) and computed automated tomography (CAT). These two techniques are eminently successful in measuring buried three-dimensional structures in the human body, and are routinely applied around the world. NMR relies on introducing a magnetic modulation in the molecular and atomic structure of certain elements in the body, and observing the response. It uses the geometric intersection of a plane and a line to determine the three-dimensional structure of the object under study. Computed automated tomography uses a series of projected measurements that are line-integrals through the object under study to de-convolve the original structure.
Wavefront sensing is a line-of-sight measurement technique. The principles of computed automated tomography may be applied to reconstruct the three-dimensional structure of an object from multiple views or measurements of the object obtained by wavefront sensing. This technique has been applied to measure three-dimensional structures in a fluid jet. To this end, eight linear wavefront sensors have been employed to simultaneously acquire high-speed data. A full three-dimensional flow field of the dynamic turbulent jet was reconstructed using this technique (see L. McMackin, B. Masson, N. Clark, K. Bishop, R. Pierson, and E. Chen, Hartmann Wave Front Sensor Studies of Dynamic Organized Structure in Flow Fields, AIAA JOURNAL, 33 (11) pp. 2158-2164 (1995)).
However, many extensions, variations and extrapolations to the system and techniques employed for measuring the fluid jet are required in order to measure a living eye or other optical system.
In Liang, et al., Hartmann-Shack Sensor as a Component in Active Optical System to Improve the Depth Resolution of the Laser Tomographic Scanner, SPIE 1542, pp. 543-554 (1991), use of adaptive optics to improve the resolution of an instrument used to make measurements near the retina is reported. A laser tomographic scanner is used to measure the retina. However, the wavefront sensor and adaptive optics system of Liang, et al. is only employed to improve the resolution of the scanner.
Accordingly, it would be advantageous to provide a system capable of measuring not only the aberrations of the individual structures, but also the full three-dimensional structure of the eye or other optical system. It would also be advantageous to provide a method of measuring the aberrations of the full three-dimensional structure of the eye or other optical system. Other and further objects and advantages will appear hereinafter.
The present invention comprises a method and system for performing optical system measurements that overcome at least one of the above disadvantages.
It is an object of this invention to determine the three dimensional structure of the eye or other optical system. This may be realized by projecting multiple spots onto a retina in such a manner that the wavefront aberration resulting from each spot may be separately determined, either simultaneously or sequentially. This group of wavefront aberration maps may then be analyzed using the methods of computed automated tomography to determine the three dimensional structure of the eye or other optical system. These and other objects of the present invention will become more readily apparent from the detailed description given hereinafter.
In one aspect of the invention, a tomographic wavefront analysis system comprises a projection system creating a plurality of collimated light beams; an optical imaging system receiving the plurality of collimated light beams and simultaneously providing the plurality of collimated light beams onto a plurality of different locations in an eye; and a wavefront sensor simultaneously receiving scattered light from each of the different locations.
In another aspect of the invention, a method of measuring aberrations of a three-dimensional structure of an optical system includes creating a plurality of collimated light beams, simultaneously providing the plurality of collimated light beams onto a plurality of different locations in the optical system, and simultaneously receiving scattered light from each of the different locations.
In yet another aspect of the invention, a tomographic wavefront analysis system comprises a projection system creating a light beam and scanning the light beam in a plurality of desired directions, an optical imaging system receiving the scanned light beam and providing the scanned light beam onto a plurality of different locations in an eye, and a wavefront sensor receiving scattered light from each of the different locations.
In still another aspect of the invention, a method of measuring aberrations of a three-dimensional structure of an optical system includes creating a light beam and scanning the light beam in a plurality of desired directions, providing the scanned light beam onto a plurality of different locations in an optical system, and receiving scattered light from each of the different locations.
In a further aspect of the system, a wavefront sensor for a wavefront analysis system, comprises a lenslet array receiving and focusing scattered light and a plurality of detector arrays located at different detector planes for detecting the focused scattered light from the lenslet array, wherein each of the detector arrays is color-coded to substantially detect only light corresponding to a different corresponding wavelength.
In a still further aspect of the invention, a lenslet array receiving and focusing scattered light and a detector array having a mosaic pattern of color-coded pixels detecting the focused scattered-light.
In yet another aspect of the invention, a corneal topography measurement can be incorporated along with the tomographic wavefront analysis system. The corneal topographer can be any of several designs, the placido disc system being the representative type of that system. The mathematical reduction of the data uses data from placido discs or the light emitting diodes or other arrangement to determine the surface shape of the cornea. Then the tomographic wavefront analysis system is used to mathematically determine the internal three dimensional structures of the eye.
However, it should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.